Mapping objects in the brain

The ability to recognize objects in the environment is mediated by the brain’s ability to integrate and process massive amounts of visual information. A research group led by Takayuki Sato and Manabu Tanifuji from the RIKEN Brain Science Institute has now discovered that in macaque monkeys, this remarkable ability is supported by mosaic-like structures in the anterior inferior temporal (IT) cortex, where localized clusters of neurons encode different visual features in an organized hierarchy. 

Two competing models have been proposed to explain the functional organization of brain regions that underlies object recognition in primates. One model states that discrete brain ‘modules’ process stimuli from particular categories, such as faces, with object recognition arising from communication among the modules. The other model postulates that the visual cortex extracts generic features, which are then composited to recognize specific objects. Since both models are based on measurements of functional signals produced by metabolic changes associated with neural activity rather than measurements of the neuronal activity itself, the precise underlying mechanism responsible for object recognition has remained unclear.

To resolve this debate, the researchers undertook dense electrophysiological mapping of neural activity in anesthetized macaque monkeys exposed to a series of color images from different object categories: faces, hands, bodies, food and various other objects. Sato and his colleagues directly recorded neuronal activity from multiple locations within the anterior IT cortex, which allowed them to track the location of neurons that responded to a particular object category.

The team found that some regions responded best to faces and others to monkey bodies. While there were also regions that responded worst to faces, none appeared to respond preferentially to hands, food or manufactured items.

Interestingly, small neuron clusters within a region appeared to be selective to different facial features, responding differently to human and monkey faces and to scrambled and normal faces. This indicates that a region in the anterior IT cortex that is selective for an object category consists of smaller-scale neuron clusters that are selective for particular visual features.

“The cortical mosaics that encode visual information seem to be efficient functional structures where object-category information and information about constituent features are represented within the limited space of the brain,” explains Sato. “This could also be the way that the brain organizes information in other sensory modalities, such as hearing.” If the results are also found to extend to humans, they may offer insight into the visual recognition of objects and the development of language.

New study reveals insight into how the brain processes shape and color

A new study by Wellesley College neuroscientists is the first to directly compare brain responses to faces and objects with responses to colors. The paper, by Bevil Conway, Wellesley Associate Professor of Neuroscience, and Rosa Lafer-Sousa, a 2009 Wellesley graduate currently studying in the Brain and Cognitive Sciences program at MIT, reveals new information about how the brain’s inferior temporal cortex processes information.

Located at the base of the brain, the inferior temporal cortex (IT) is a large expanse of tissue that has been shown to be critical for object perception. This region of the brain is commonly divided into posterior, central, and anterior parts, but it remains unclear as to whether these partitions constitute distinct areas. An existing, popular theory is that the parts represent a hierarchical organization of information processing, a notion that has previously been supported by functional magnetic resonance imaging (fMRI) in monkeys. For their study, Conway and Lafer-Sousa used non-invasive fMRI to measure responses across the brains of rhesus monkeys to a range of different stimuli and obtained responses to images of objects, faces, places and colored stripes. “The technique enabled us to determine the spatial distribution of responses across the brain, and has been useful in figuring out how the visual brain is organized,” Conway said.

Conway, a visual neuroscientist and artist, examines the way the nervous system processes color using physiological, behavioral, and modeling techniques. Conway and Lafer-Sousa assert that color provides a useful tool for tackling questions about processing in the IT region, as it has little “low-level” feature similarity with shapes (psychological work shows that color can be perceived independent of shape)—therefore any relationship between color-responsive and shape-responsive regions should reflect fundamental organizational principles.

"Shape and color are both properties of objects and are processed by the parts of the brain known to be important for detecting and discriminating objects. However, the way this part of brain is organized has not been clear, for example, is color computed by different parts of this region than those that compute shape?" The answer to this question, Conway said, has deep implications for understanding the general computational principles used by the brain and how the brain evolved.

"Our work showed that, to a large extent, color and faces are handled by separate, parallel streams, and that these pieces of information are processed by connected, serial stages," Conway said. "One can imagine the processing as an assembly line, where some aspect of faces – and some aspect of color – is computed first. The output is then sent to another region downstream that makes a subsequent computation."

They hypothesized that the earliest stages in color processing involve detecting and discriminating hue, while the later stages compute color-memory association. For example, the brain may first compute that yellow is diagnostic of banana, then later, color categories are recognized; for example, limes, grass, and fern leaves are all “green.”

"The most striking aspect of the study is what it reveals about the precision of the organization of the brain. We often think that because the brain consists of billions of neurons, that at some level it must be quite variable how the neurons are organized," Conway said. "The study shows that there is a remarkable precision in organization of the neural circuits for high-level vision, which will make tractable many questions bridging cognitive science and systems neuroscience."

As a visual artist, Conway said the aspect of the research he finds most satisfying is the beauty of the organizational patterns that, he said, are “clearly are the result of a set of underlying organizational principles.” He continued, “It is interesting to think that the brain reflects what artists have long recognized: that color and shape can be decoupled, each represented somewhat independently—think of color monochromes versus black-and-white line drawings. The neural architecture provides a reason why this is effective or possible.”

The researchers note that it remains unclear whether the organizational principles found in humans apply to monkeys, an important issue that bears on cortical evolution. However, their results suggest that the IT comprises parallel, multi-stage processing networks subject to one organizing principle.